The connection by which the artificial joint is held in the host bone is of critical importance to the long term success of any joint replacement surgery. The characteristics of the artificial joint must be matched with the characteristics of the naturally occurring host bone to prevent undue stress resulting in trauma to the natural bone and pain to the host.
Bone prosthesis are either secured in the host with bone cement or without the use of cement. While initially providing a useful connection between the prosthesis and the bone, the earlier cements tended to degrade over time with deleterious results. One approach to overcome this degradation problem was to not use cement at all. This invention relates to installing joint prosthesis without the use of bone cement.
When no cement is used, the connection or fit of the prosthesis within the intramedullary canal of the natural bone becomes more critical. Without the cushioning effect of cement surrounding the intramedullary insert of the prosthesis, there is a requirement for a more accurate matching of the prosthesis insert with the host bone. The prosthesis must, not only, conform to the size of the intramedullary canal of the host bone but to the inherent strength and flexibility or brittleness of the cancellous bone surrounding the intramedullary canal.
These different criteria can create mutually exclusive design requirements, for example, a large insert that must be very flexible because the characteristics of the host bone or a small insert that must be very stiff.
There are various patents and publications directed to the implantation of bone prosthesis without the use of cement each citing the attendant advantages of such procedures. Several patents are directed to the flexibility of joint prosthesis.
U. S. Pat. No. 5,549,702 to Ries et al discloses a one piece or two piece prosthesis in which one end or one portion of the prosthesis is inserted into the intramedullary canal of the host bone. The intramedullary insert of Ries et al has various modifications to achieve a specific flexibility. The insert may be a solid bar or it may be partially tubular. Among the modifications are spiral slots of varying depth. The slots may extend partially through the insert or may extend from a central bore through the entire thickness of the insert forming a coil structure. Other modifications include varying the thickness of the insert along the longitudinal axis. Once each of these variations is manufactured, the flexibility of the insert is permanently set.
U.S. Pat. No. 4,997,444 to Farling discloses a joint prosthesis in which the modulus of elasticity of the intramedullary insert may be varied along the length. The variation results from stacking disks having different characteristics upon a central strut or struts. Farling states that the disks and struts are connected together mechanically or metallurgically bonded together. This indicates that these devices are manufactured with various degrees of stiffness throughout their length and the resulting device cannot or should not be changed thereafter.
U.S. Pat. No. 5,062,851 to Branemark discloses a joint mechanism to be used in smaller bones. The tubular anchoring element has external screw threads for bone contact and a tapering wall thickness. The end portions of the tubular element have longitudinal slits and the wall thickness decreases toward the slit portion. The combination of the slits and thinner walls produce a more flexible end portion of the device.
These references are merely illustrative of the prior art and are not to be considered to be an exhaustive listing.